1. Field of the Invention
This invention relates to a master information carrier used for static and areal lump-sum recording of digital information signals on a magnetic recording medium.
2. Description of the Related Art
A magnetic reading and reproducing apparatus has been increasing in recording density to realize a small size and large capacity. Especially, in the field of a hard disk drive as a typical magnetic recording device, an a real recording density of more than several gigabits per square inch is already available on the market. Further an areal recording density of ten gigabits per square inch is expected in few years.
One of the primary factors that has enabled such high recording density is the increasing linear recording density, due to improvements of medium properties, head-disk interface performance, and a new signal processing method such as “partial response”. However recently the rate of increase-in track density exceeds that of linear recording density, and thus becomes a primary factor for increasing areal recording density. Practical use of a magneto-)resistive type head, which is superior to a conventional inductive type head in reproduction output performance, has contributed to the progress in the track density. It is possible at present to read a signal from a track whose width is at most only a few microns with-good S/N ratio by practical use of the magneto-resistive type head. Further it is expected that a track pitch will reach the sub-micron range in the near future along with further improvement of the head performance.
A tracking servo technique is important for the head to read a signal with high S/N ratio by scanning precisely such a narrow track. For example, a conventional hard disk has areas that are located at predetermined angular intervals over 360 degrees. In those areas information such as a tracking servo signal, address and clock signal is provided (hereinafter referred to as ‘preformat’). A magnetic head can scan a track by reading such information at predetermined periods, and monitoring and correcting the head position.
The above-mentioned tracking servo signal, address and clock signal are to be reference signals for the head to scan a track precisely. Therefore, precise record positions are required for these information signals. Current preformat recording into a hard disk is performed precisely by magnetic heads placed in the hard disk drive by using a special servo-track recording apparatus after installing the disk into the drive.
The above-mentioned preformat recording using such a special servo-track recording apparatus has some problems as follows.
The first problem is due to the fact that relative movement between the head and the recording medium is necessary for recording with the magnetic head. This fact means that a substantially long period is required for preformat recording. In addition, the special servo track recording apparatus is expensive. Thus, the cost for preformat recording is quite high.
The second problem is that due to a space between the head and a medium or due to a diffusive recording magnetic field caused by a pole shape of the recording head, the magnetic transition at track edges of the recorded preformat signals lacks steepness. In a current tracking servo technique, the head position is detected by the amount of change in a read signal amplitude when the head missed a track. Therefore, the system requires a steep off-track performance, in which reproduced signal amplitude changes sharply as the head misses the track The diffusive recording magnetic field acts against this requirement, and thus, makes it difficult to realize a precise tracking servo technique that is required for a submicron track recording.
In order to solve the above-mentioned problems in preformat recording with a magnetic head, Japanese Laid-open Patent Application Tokkai Hei) 10-40544 discloses a new preformat recording technique. In the disclosure, a master information carrier comprising a substrate having an embossed pattern on it is prepared The pattern corresponds to the preformat information signal. At least the protruded portion of the embossed pattern is made of a ferromagnetic material layer. By contacting the surface of the master information carrier with the surface of a magnetic recording medium and applying a magnetic field, the preformat information is recorded in the magnetic recording medium as a magnetized pattern corresponding to the embossed pattern.
According to the disclosure of Tokkai-Hei 10-40544, a ferromagnetic, material composing the protruded portion of a master information carrier surface is magnetized by the applied magnetic field. By the recording magnetic field generated from the magnetized ferromagnetic material, the magnetized pattern corresponding to the embossed surface is recorded on a magnetic recording medium. Thus, the preformat recording of the tracking servo signal, address information signal, read clock signal and other signals is achieved by using the embossed pattern formed on the surface of the master information carrier.
While relative movement between the head and the medium is required for conventional linear recording with a head, the technique of Tokkai Hei 10-40544 is characterized by a static and areal lump-sum recording that does not require relative movement between a master information carrier and a medium. As a result, the technique disclosed in the reference is generally effective for the problems related to preformat recording as follows:
First, the time needed for the preformat recording is substantially shorter as compared to the prior art using a magnetic head. In addition, an expensive servo-track recording apparatus is not necessary for precise position control of the magnetic head. Therefore, the technique disclosed in the reference can improve the productivity of the preformat recording and reduce production costs.
Secondly, a space gap between the master information carrier and the magnetic recording medium can be minimized, since relative movement between them is not required for recording the information signal. In addition, the recording magnetic field for recording does not diffuse, unlike the prior art using a magnetic head. Thus, the magnetic transition at track edges of the recorded preformat signal is steep compared with the recording with a magnetic head. This ensures a precise tracking of a magnetic head in reading data signals from the magnetic recording medium.
In the signal recording process of this technique, the master information carrier and a magnetic recording medium should be contacted with each other securely and uniformly over a large area. Tokkai-Hei 10-269566 discloses a specific recording apparatus to meet this requirement with a function of sucking air between the master information carrier and the magnetic recording medium to secure the contact between them with the pressure of the surrounding atmosphere.
Tokkai-Hei 10-40544 discloses a master information carrier comprising a substrate on which an embossed pattern corresponding to information signals is formed precisley by means of photolithography or the like, and at least the protruded portion of the embossed surface is made of a ferromagnetic material. The master information carrier, however, will be subjected to partial stress intermittently and repeatedly when the preformat recording is performed from the process of sucking air between the master information carrier and magnetic disks being repeated to contact them securely under the pressure of the surrounding atmosphere, by using the recording apparatus disclosed in Tokkai-Hei 10-269566.
Specifically, as the ferromagnetic material at the protruded portion contacts directly and repeatedly with the magnetic disks, the ferromagnetic material will be chipped gradually to lose accuracy in the embossed shape. When the chipping of the ferromagnetic material becomes serious, the recording signals will be lost or the magnetic disks will be damaged.
In view of these facts, the master information carrier disclosed in Tokkai-Hei 10-40544 requires improved durability. The master information carrier is required to allow repetition of good preformat recording without losing recording signals or damage to the magnetic disks, i.e., the master information carrier should have a long life, because the life affects the number of recordings that can be made using the master information carrier.